Recently, there have been developed such display devices as: a display device including photodetectors in a frame region surrounding a display screen; and a display device including a plurality of photodetectors each of which is provided, at a certain interval, for a display region of a display panel having a plurality of pixels, and is included within corresponding one of the plurality of pixels. Such display devices can make use of photodetection functions of the photodetectors so as to realize functions such as a function to perform light control to a backlight, a touch-panel function, an OCR function for character recognition, or a security function including fingerprint authentication.
An example of the photodetectors to be included in the display devices includes a PIN photodiode. The PIN photodiode has a configuration which is classified into a longitudinal configuration or a lateral configuration. In the longitudinal configuration, a P-type layer, an I-type layer, and an N-type layer are laminated in this order onto a substrate. In the lateral configuration, a P-type layer, an I-type layer, and an N-type layer are provided in a plane direction on a substrate. Note that the P-type layer is a semiconductor layer having high P-type impurity concentration, the I-type layer is an intrinsic semiconductor layer or a semiconductor layer having low impurity concentration, and the N-type layer is a semiconductor layer having high N-type impurity concentration.
The lateral configuration, out of these configurations, is a configuration in which the P-type layer, the I-type layer, and the N-type layer do not overlap one another. Thus, parasitic capacitance formed among these layers is small. This gives the lateral configuration a merit in that it has a sensing speed faster than that in the longitudinal configuration. Also, the lateral configuration has another merit in that it can be manufactured by a same process as that for manufacturing a TFT having a configuration such as PNP, PIP, NPN, or NIN.
FIG. 17 shows a photodetector which is a PIN photodiode having a lateral configuration (see Patent Literature 1 below). A photodetector 81 in FIG. 17 has a P-type layer 82, an I-type layer 83, and an N-type layer 84 which are formed onto a silicon film 85. The P-type layer 82, the I-type layer 83, and the N-type layer 84 are formed, in this order, along a plane direction of the silicon film 85. The silicon film 85 is formed on a glass substrate 90 serving as a base of an active matrix substrate. Also, the P-type layer 82 is connected to an electrode pattern 88 via a contact plug 86, and the N-type layer 84 is connected to an electrode pattern 89 via a contact plug 87.
When a reverse bias voltage is applied across such photodetector 81 and the photodetector 81 on this condition receives light, an electric charge is generated in amounts according to intensity of the light irradiation. A resultant current flows from the electrode pattern 89 to the electrode pattern 88.
Manufacturing of the PIN photodiode having the lateral configuration includes, for example, (i) covering the silicon film 85 by a mask having a pattern for preparing the P-type layer 82, and performing thereonto ion implantation of P-type impurities including boron, and (ii) covering the silicon film 85 by another mask having a pattern for preparing the N-type layer 84, and performing thereonto ion implantation of N-type impurities including phosphor and arsenic.
In order that the photodetector 81 may generate a larger current due to light reception, it should be employed to increase a size W (see FIG. 17: hereinafter referred to as a channel width W) of longitudinal sides of the I-type layer 83 by which the I-type layer 83 is in contact with the P-type layer 82 and the N-type layer 84. Note that a relation between a size L (hereinafter referred to as a channel length L) of a transverse side of the I-type layer 83 and an intensity I of the current is as shown I FIG. 18.
That is, as shown in FIG. 18, as the channel length L is increased from 0, a current I is increased in a range to Lp at which recombination of an electron hole and electron in the I-type layer 83 is greater than generation thereof. Thus, the amplitude I is increased in a range where 0<L<Lp, becomes maximum at L=Lp, and is decreased in a range where Lp<L due to recouping.